There is an ever increasing demand for higher data rates in cellular networks, which poses challenges to developers of such wireless networks. The existing cellular networks should preferably be evolved in a cost efficient as well as time efficient manner so as to meet the requirements for higher data rates. In this respect, a number of approaches are possible. A first option is to increase the density of existing base stations; a second option is to increase the cooperation between the base stations and a third option is to deploy smaller base stations in areas where high data rates are needed within the existing base station grid. The use of smaller base stations within the current macro base stations network is referred to as “heterogeneous networks”, or multilayer network, and a layer consisting of smaller base stations is termed a “micro” or “pico” layer. The overlaying base stations are then called macro base stations.
Building a denser macro base station grid and simultaneously enhancing the cooperation between the macro base stations (i.e. first and second options above) is a solution that certainly would meet the requirements for higher data rates. However, such an approach is not necessarily a cost efficient approach, as both costs and delays associated with installation of macro base stations are significant, especially in urban areas.
The third option, i.e. deploying small base stations within the already existing macro layer grid is an appealing solution, since these smaller base stations are anticipated to be more cost efficient than macro base stations, and their deployment time will be shorter as well. However, such a dense deployment of macro and micro base stations would lead to a significantly higher amount of signaling due to frequent handovers for users moving at high speed.
The macro layer grid of the heterogeneous network could serve users moving at high speed, as well as service wider areas where the demand for high data rates is less and the grid consisting of smaller base stations in the heterogeneous network could be employed in service areas having a higher density of users requiring high data rates, or hotspots as such areas are denoted. In the heterogeneous network thus, macro base stations could be used for coverage and micro or pico base stations for capacity. This provides a semi-static or dynamic sharing of resources across macro-micro/pico layers.
One of the main targets of micro base stations, also denoted low power nodes, is to absorb as many users as possible from the macro layers. This would offload the macro layer and allow for higher data rates both in the macro and in the micro layer. In addition, users would have increased radio quality, especially in uplink, than when connected to macro base stations, since they are generally going to be closer to smaller base stations.
In this respect, mainly two techniques have been discussed within the 3GPPP:
1) extending the range of small cells by using cell specific cell selection offsets, and
2) increasing the transmission power of low power nodes and simultaneously appropriately set the uplink power control target P0 for the users connected to low power nodes.
The former method seems to attract more interest of standardization for 3GPP, mainly due to the higher flexibility that it offers. Increasing the power of low power nodes has limits related to the maximum transmission power of the low power nodes.
By applying any of the above techniques the interference in the downlink control channels increases. Since downlink control channels are transmitted over the whole bandwidth, classical inter-cell interference (ICIC) mechanisms cannot be applied to them.
This creates new interference scenario of a given cell receiving high other cell interference in the downlink control channel by a neighbor cell transmitting with higher power. The main technique which has been investigated by the 3GPP Long term evolution (LTE) standardization process is to employ “Almost Blank Sub-frames” (ABS) at the macro layer. The macro layer is muted so as to not create high other cell interference to users that are both connected to the low power nodes and are located at an extended range thereof. The coverage range of a low power node can be increased by using a cell selection offset (or handover thresholds), and this increased coverage range is denoted extended range of low power node.
ABS is a technique which does solve the problem of interference generated by the macro layer to users connected to low power nodes and located at the extended range of the low power nodes. However, a drawback of this technique is that resources are not fully used at the macro layer. Moreover, for the case in which the macro layer is heavily loaded and the number of micro layer users located at the extended range is low, a high number of macro layer users will have to underutilize their resources so as not to interfere with few micro layer users located at the extended range of the low power nodes. This is an inefficient use of radio resources and might become even more pronounced if the users connected to low power nodes and located at the extended range of small cells are having relatively good link to the macro base station and not very much uplink data to send. Hence the benefit from connecting to a base station yielding better uplink connection is not considerable, but the amount of radio resources sacrificed for this benefit is high. From the above, it is clear that it is a challenging task to balance different aspects, such as use of communication resources and minimizing interference, when optimizing the performance of the communication system. That is, underutilization of communication resources obviously affects the service that can be offered to users in a similar way the service offered to users is affected by interference. It is clear that there is room for improvements on this situation in this field of technology.